1. Field of the Invention
The present invention relates to the field of voice and data communications, and more particularly, unilateral and bilateral communications not depending on human intervention at both ends of the communication link.
2. Prior Art
Various types of communication systems are well known in the prior art. Such systems generally fall into three categories, specifically, unilateral or bilateral systems for communication between between two specific points, systems having a central transmitter and a plurality of receivers for broadcasting from the central point for reception as desired by any of the remote receivers, and systems having a plurality of transmitters for unilateral or bilateral transmission to a central receiver. Typical of the first type of communication systems are microwave systems, of the second type are standard AM, FM and television broadcast systems, and of the third type are police communication systems for communicating between various police cars and a central station. Of particular interest to the present invention is the second type, wherein information is to be transmitted from one, or at least a small number of transmitters, to a relatively large plurality of receivers.
More particularly, there is a need for communication systems to allow prompt publication of public alert and advisory information in specific types of emergencies, such as by way of example, in the event of an accident in a nuclear power plant which may allow release of radioactive materials into the atmosphere. By way of specific example, the Nuclear Regulatory Commission may soon require that 100% of the people located within five miles of a nuclear power plant be warned within 15 minutes of a nuclear accident, and that 95% of the people within five to ten miles of the power plant be similarly warned within 15 minutes. At the present time the primary system which might be used in an attempt to meet these requirements is the Emergency Broadcast System (EBS) which provides any required warning messages by interruption of conventional AM broadcast programming. This, of course, has the advantage of not requiring any special equipment at either the transmitter end (except perhaps for special lines for directing emergency messages to the transmitter) or at the receiving end. However, it has a number of disadvantages which make its usefulness highly limited. Effective communication requires that the AM receiver be both turned on and tuned to a station carrying the emergency broadcast. However, statistics show that most AM receivers are turned off most of the time (nuclear accidents can occur at any time during the day or night). Also, since the system depends upon interruption of normal AM broadcast programming, testing of the system, and particularly the testing of the effectiveness thereof is subject to great limitation. Further, in general antennas for conventional AM broadcast stations are not located at the site of nuclear power plants and do not have a range limited to ten miles, but instead commonly have random positions many miles from a nuclear power plant, and are effective at all for the EBS only because they serve an area of a radius of many tens of miles which encompasses the required region surrounding the nuclear power plant. Accordingly, the public alert information which is broadcast is broadcast not only to the area to be alerted, but to the entire area served by the AM station, partially defeating the purpose of the system as a result of the attraction of at least some people toward the area of emergency out of curiosity. In addition, while alert information can be broadcast, i.e., information regarding the nature of the accident or other public emergency, specific advisory information cannot, as the same advisory information is brought to all people in the area. In particular, while general evacuation instructions can be given, i.e., the location away from which people should evacuate, specific information directing people in some locations to evacuate via a certain route, and other people in other locations to evacuate via other routes, is more difficult to broadcast, as everyone in the broadcast area receives everyone's instructions, which at best will be a highly confusing situation. Thus, it is obvious that an ideal system would include the ability to broadcast and receive the required information at remote locations without the receiver being manually turned on, and would further include the ability to transmit specific instructions for various areas within the emergency region, with the people in any specific area within the emergency region only receiving that information applicable to them, and with none of the people outside of the emergency region even receiving the basic warning itself.
Other possibilities for such communication of course, would include the National Oceanagraphic and Atmospheric Administration broadcast system (NOAA), which is a 24 hour broadcast system, or for that matter, any other dedicated broadcast system for such purpose, though the requirement of a dedicated broadcast station and special receivers (fixed tuning and always on) is economically unattracative and does not provide the advantageous selective communication feature.
U.S. Pat. No. 4,117,405 and U.S. Pat. No. 4,208,630 entitled "Narrow Band Radio Communication System" and "Narrow Band Paging or Control Radio System", respectively, describe communication systems which allow the communication of information by the narrow band phase modulation of the carrier of a conventional AM broadcast station. In particular, in accordance with those patents, if one modulates the phase of the carrier of a conventional AM broadcast station within certain limits, information may be broadcast on the carrier of the AM station without detection by or significant interference with the operation of conventional AM radio receivers. Depending upon the limits of the phase modulation, compatible limits on the spectrum of the phase modulation may be imposed so that data may be communicated by the modulation of the carrier of the AM station at meaningful rates without detection by conventional AM receivers, the patents disclosing, by way of example, a phase modulation rate of 18 Hz and a phase modulation of plus or minus 15 degrees as being suitable for the purpose. In addition, the patents disclose a form of frequency synthesizer which may be used at the receivers to accurately reconstruct the unmodulated carrier frequency against which the phase changes can be measured, and to generate various other frequencies for various purposes. In U.S. Pat. No. 4,208,630, the AM carrier is being used to synchronize other transmitters and receivers in this manner, and to put out hourly time beep synchronization signals which are detected and used at both the control transmitter and at the plurality of receivers so as to synchronize the digital data pulses therebetween, and thereby precisely establish the time at which digital message bits will be transmitted and received.
An article entitled "Minimum-Bandwidth Multiplex Radio Teleprinter System" appearing in the June 1971 publication of the IEEE Transactions on Communication Technology described a data system using AM broadcast signals to carry teleprinter signals. Usable at medium or short wave frequencies, the multiplex technique provided point to point or broadcast teleprinter service utilizing existing transmitters and antennas. A minimum bandwidth phase modulation "subcarrier" was used to phase modulate the broadcast carrier, proportedly without disturbing operation of either the broadcast transmitter or receiver. The system used separate crystal oscillators at the source and at the receiver so that the data streams were not synchronized at the two locations, thereby requiring a transmitted identity for both the beginning of a digit and the beginning of a word. Such a system has not been widely used however, neither for radio teleprinter nor other communication, probably because of its low speed, lack of synchronization between the transmitter and receiver and nonselective communication characteristic thereof.